This invention relates to an electromagnetic induction heating apparatus which can prevent undesirable states of cooking utensils or vessels and, more particularly, to an apparatus for heating an object to be heated such as a cooking vessel based on an eddy current loss caused by electromagnetic induction.
In a conventional induction heating apparatus, an induction heating coil is arranged below a top plate on which an object to be heated such as a cooking vessel is placed. A high-frequency current is supplied to the induction heating coil by an inverter, so that a high-frequency magnetic field is applied to the cooking vessel to flow an eddy current therethrough, thereby heating it.
In the induction heating apparatus of this type, since a current flowing through the induction heating coil has an opposite phase to the eddy current flowing through the cooking vessel, the cooking vessel receives a repulsion effect. If the cooking vessel is made of a magnetic material such as iron, the cooking vessel is attracted by the magnetic force caused by the magnetic field from the coil, and as a result, a repulsion force acting on the cooking vessel is reduced.
However, if the cooking vessel is formed of a non-magnetic material such as aluminum (Al), an attractive force due to the magnetic force is small. Since an Al cooking vessel has a small specific permeability and surface resistance, a large eddy current must be flowed through the Al vessel in order to make the input resistance with respect to the induction heating coil equivalent to that of an iron cooking vessel. Therefore, a repulsion force acting on the Al cooking vessel undesirably increases. As a result, if a total weight of the Al vessel and a material to be heated stored therein is small, the vessel often floats above the top plate. If this state is left unchanged, the heating efficiency is considerably reduced, and sufficient induction heating cannot be performed. In the worst case, the vessel is moved along the top plate.
FIG. 5 shows the relationship between a power [W] of the inverter and a repulsion force [g] acting on the vessel. In this case, the vessel is formed of Al, the number of turns of the induction heating coil is 80 turns, the frequency is 50 KHz, and a distance between the vessel and the heating coil is 6 mm. As can be seen from FIG. 5, the repulsion force is increased in proportion to the power of the inverter. More specifically, as the output increases, the vessel tends to float from the top plate.
In order to solve the above problem, a detection mechanism is necessary to detect whether or not the vessel is floating from the top plate. For example, in a detection technique, a detection element for detecting a change in magnetic flux density, such as a Hall element or a search coil, can be used. In this case, a change in magnetic flux density cannot often be detected, depending on the location of the detection element, resulting in poor detection reliability. In this technique, since a magnetic flux density to be detected changes in accordance with the power of the inverter, a detection circuit arrangement becomes complex.